Oxidants are produced as part of the normal metabolism of all cells but also are an important component of the pathogenesis of many disease processes. Reactive oxygen species, for example, are critical elements of the pathogenesis of diseases of the lung, the central nervous system and skeletal muscle. Oxygen free radicals also play a role in modulating the effects of nitric oxide (NO.multidot.). In this context, they contribute to the pathogenesis of vascular disorders, inflammatory diseases and the aging process.
A critical balance of defensive enzymes against oxidants is required to maintain normal cell and organ function. Superoxide dismutases (SODs), a family of metalloenzymes which catalyze the intra- and extracellular conversion of O.sub.2.sup.- into H.sub.2 O.sub.2 plus O.sub.2, and represent the first line of defense against the detrimental effects of superoxide radicals. Mammals produce three distinct SODs. One is a dimeric copper-and zinc-containing enzyme (CuZn SOD) found in the cytosol of all cells. A second is a tetrameric manganese-containing SOD (Mn SOD) found within mitochondria, and the third is a tetrameric, glycosylated, copper- and zinc-containing enzyme (EC-SOD) found in the extracellular fluids and bound to the extracellular matrix. Several other important antioxidant enzymes are known to exist within cells, including catalase and glutathione peroxidase. While extracellular fluids and the extracellular matrix contain only small amounts of these enzymes, other extracellular antioxidants are known to exist, including radical scavengers and inhibitors of lipid peroxidation, such as ascorbic acid, uric acid, and .alpha.-tocopherol (Halliwell et al, Arch. Biochem. Biophys. 280:1 (1990)). The relative lack of extracellular antioxidant enzymes may reflect the possible function of extracellular reactive oxygen species as bioeffector molecules (Halliwell et al, Arch. Biochem. Biophys. 280:1 (1990)). The relative deficiency of such enzymes may also result in greater susceptibility to extracellular oxidant stresses.
The enzyme EC-SOD, in many extracellular locations, exists only at low concentrations. While its physiologic role in vivo is yet to be defined, in many extracellular locations, EC-SOD is not thought to function as a bulk scavenger of O.sub.2.sup.-. As indicated above, EC-SOD is a tetrameric Cu/Zn-containing glycoprotein with a subunit molecular weight of 30,000 (Marklund, Proc. Natl. Acad. Sci. USA 79:7634 (1982); Tibell et al, Proc. Natl. Acad. Sci. USA 84:6634 (1987); see also U.S. Pat. No. 5,130,245 and WO 91/04315). Biochemical data suggest that EC-SOD binds to heparan sulfate proteoglycans on endothelial cells, where it has been speculated to serve as a "protective coat" (Marklund, J. Clin. Invest. 74:1398 (1984); Karlsson et al, Biochem. J. 255:223 (1988)). Endothelial cells secrete both O.sub.2.sup.- (Halliwell, Free Radical Res. Commun. 5:315 (1989)) and endothelium-derived relaxing factor, putatively identified as nitric oxide (NO.multidot.) (Noak and Murphy, in Oxidative Stress Oxidants and Antioxidants, eds Sies, H. (Academic, San Diego), pp. 445-489 (1991)). NO.multidot. functions as a vasoregulator and as a regulator of neurotransmission (Schuman and Madison, Science 254:1503 (1991)). NO.multidot. can, however, be toxic to neurons in some situations (Dawson et al, Proc. Natl. Acad. Sci. USA 88:6368 (1991)). O.sub.2.sup.- is known to inactivate NO.multidot. -induced vasorelaxation (Gryglewski et al, Nature 320:454 (1986); Rubanyi and Vanhoutte, Am. J. Physiol. 250:H822 (1986); Rubanyi and Vanhoutte, Am. J. Physiol. 250:H815 (1986); Bult et al, Br. J. Pharmacol. 95:1308 (1988); Nucci et al, Proc. Natl. Acad. Sci. USA 85:2334 (1988)). Thus, a possible function for EC-SOD is to protect NO.multidot. released from cells from O.sub.2.sup.- -mediated inactivation.
The reaction of O.sub.2.sup.- with NO.multidot. is also known to produce a potentially toxic intermediate in the form of the peroxynitrite anion (ONOO.sup.-) (Beckman et al, Proc. Natl. Acad. Sci. USA 87:1620 (1990); Mulligan et al, Proc. Natl. Acad. Sci. USA 88:6338 (1991); Hogg et al, Biochem. J. 281:419 (1992); Matheis et al, Am. J. Physiol. 262:H616 (1992)). Thus EC-SOD may also function to prevent the formation of ONOO.sup.-.
Surprisingly, it has been found that EC-SOD increases, rather than decreases, central nervous system O.sub.2 toxicity and that this effect of EC-SOD occurs through modulation of NO.multidot. This result implicates NO.multidot. as an important mediator in O.sub.2 toxicity. The invention thus relates to methods of manipulating nitric oxide function that involve the use of extracellular antioxidants.
In addition to superoxide radicals, hydrogen peroxide is an oxidant species that is produced under a wide variety of conditions of oxidant stress. The invention thus also provides a method of manipulating hydrogen peroxide levels.
The methods of the invention find application in various disease and non-disease states in which oxidative stress plays a role, including inflammation. In a broader sense, the invention relates generally to methods of modulating intra- and extracellular processes in which an oxidant such as O.sub.2.sup.- or hydrogen peroxide is a participant.